A lighting device is an apparatus which radiates that light from a planer light emitting surface, which is emitted from a light source. Such a lighting device is not only solely used as a lighting device but also used in a liquid crystal display apparatus in combination with a liquid crystal display panel.
The lighting devices include a bulb-type lighting device each comprising a hemispherical or bulb-type light emitting surface, and a planar lighting device having a flat light emitting surface. In recent years, a light emitting diode (LED) is frequently used as a light source. A variety of lighting devices using an LED have been proposed. LEDs provide features such as easy light modulation and easy maintenance but a problem occurs when an LED is applied to a conventional lighting device.
Concerning the bulb-type lighting device, the directivity of light from an LED light source deviates in the frontal direction. Uniform light distribution is therefore difficult to achieve unlike a conventional incandescent bulb or fluorescent bulb. Hence, a measure has been proposed to achieve a uniform light distribution by using, as a lighting device which achieves the required light distribution, an LED board in the shape of a polyhedron.
Lighting devices are categorized into a directly-under type in which light sources are arranged in a plane directly under a light emitting surface, and a side type in which light sources are arranged on a side of a plate-type light guide member. The directly-under-type-planar lighting device achieves excellent light use efficiency, and the side-type planar display apparatus has excellent thinness. Because of the high light use efficiency and simple structure, the directly-under-type lighting device is more prevalent, although thinning has been strongly demanded for the directly-under-type lighting device. In this case, the LED light source gives rise to a problem of high directivity and easily causes an uneven luminance in which the luminance increases just above the light source in a light emitting surface. Therefore, in order to uniformly equalize the luminance in the light emitting surface, the lighting device needs to be thick, which hinders thinning of the lighting device.
In respect of the bulb-type lighting device, the manufacture of a polyhedral board is complicated, and heat radiation needs to be ensured while maintaining the mechanical strength of a board supporting part. Thus, product design is highly complex. As another method, reflection of light within an outer container may be increased by using a highly diffusive material for the outer container surrounding an LED board. However, reducing the directivity of an LED light source merely via the diffusibility of the outer container is limited. There is a further problem in that the efficiency of extracting light deteriorates as the number of times of reflection within the outer container increases more than required.